This invention is directed to a low-density, open-cell, thermoplastic, absorbent foam that is soft and flexible. The foam can be made with balanced amounts of one or more surfactants and a plasticizing agent in a foam polymer formula. Thermoplastic elastomers can be added to the foam polymer formula to improve softness, flexibility, elasticity and resiliency.
Thermoplastic absorbent foam is made of polymer(s) that can be heated, formed and cooled repeatedly, typically commercially using a continuous plastic extrusion process. Thermoplastic absorbent foam can be used to produce personal care products including, but not limited to, absorbent articles such as disposable diapers, baby wipes, training pants, child-care pants, and other disposable garments; feminine-care products including, but not limited to, sanitary napkins, wipes, menstrual pads, panty liners, panty shields, tampons, and tampon applicators; adult-care products including, but not limited to, wipes, pads, containers, incontinence products, and urinary shields. Besides use of such foam for personal care products, thermoplastic absorbent foam can also be used in a wide array of applications including a variety of professional and consumer health and medical care products including, but not limited to, products for applying hot or cold therapy, hospital gowns, surgical drapes, bandages, wound dressings, wipes, covers, containers, filters, disposable garments and bed pads, medical absorbent garments, underpads, and the like, as well as clothing components, filters, thermal and acoustic insulation, shock and cushion absorbing products, athletic and recreation products, construction and packaging uses, and service, industrial, and household products including, but not limited to, cleaning applications such as sponges and wipes for oleophilic and/or hydrophilic fluids; products for cleaning and disinfecting, and covers, filters, towels, bath tissue, and facial tissue; nonwoven roll goods; home-comfort products including pillows, pads, cushions, and masks and body-care products such as washcloths, and products used to cleanse or treat the skin. Low foam density and low modulus are required for high absorbency, softness, flexibility, and in desired hand and fit aesthetics for applications such as diapers, incontinence products, and other aforementioned products.
Extruded foams have a cellular structure, with cells defined by cell membranes and struts. Struts are formed at the intersection of cell membranes, with the cell membranes covering interconnecting cellular windows between the struts. The thickness of cell struts is typically 2–10 times greater than the thickness of cell membranes. Extruded foams are typically produced with substantially closed cells. The open-cell content of closed-cell foams is generally less than 20%. Acceptable absorbent foam has an open-cell structure, typically 50% or higher, as measured by ASTM D2856, and suitably has a controlled cell diameter. Specific cell size and cellular connectivity are adjusted to the desired function, such as for high capillary fluid movement and high absorption capacity. Cell wall or membrane pores that connect cells are of sufficient number and size to minimize viscous drag and flow resistance to produce effective fluid transport and containment. Reticulated foam generally has a minimal number of cell windows or no cell windows (only struts) and, with sufficiently small enough pores, can effectively transport capillary fluid. Such open-pore structures lend themselves to rapid fluid intake.
Processes are known for making open-cell foams, low-density foams, absorbent foams, and soft, resilient, elastomeric foams. One process for enhancing the open-cell formation in foam is described, for example, in U.S. Pat. No. 5,962,545. All of these foam qualities in a single foam would be particularly desirable in a number of absorbent product applications; however, it is difficult to produce such foam.
Foaming soft, flexible polymers, such as thermoplastic elastomers, to low densities with absorbency is difficult to achieve. U.S. Pat. No. 5,728,406 describes low-density, flexible, non-absorbent foam. As described in U.S. Pat. No. 6,451,865, heat-expandable thermoplastic particles that encapsulate a heat-expandable gas or liquefied gas can be added to produce such thermoplastic elastomer foam.
Plasticizing agents are sometimes used as cell openers in producing foams. When used as cell openers, these plasticizing agents are added to the thermoplastic foam polymer formula in minor amounts, as described in U.S. Pat. No. 6,071,580. More particularly, the plasticizing agent can act to increase cell expansion to produce a high expansion ratio. When cells expand, membranes between cells thin and can become unstable, rupture, and can thereby create porous connections between cells. In addition, when thermoplastic polymer cools and with volumetric contraction with crystallization, thin portions of the membrane can rupture enough to create additional connections or pores between cells, thereby creating open cells.
Although plasticizing agents act as softeners, the addition of plasticizing agents makes foaming to low densities even more difficult. U.S. Pat. No. 6,653,360 describes a high density, essentially closed-cell, non-absorbing foam containing a plasticizing agent and thermoplastic elastomer and additive such as a surfactant. In particular, plasticizing agents typically lower polymer melt viscosities and lead to increasing melt drainage that causes foaming difficulties with cell collapse. In fact, in certain manufacturing processes, such as food packaging processes, plasticizing agents are used as defoaming agents.
There is a wide range of FDA-approved plasticizing agents available. The criterion for selecting a plasticizing agent for personal care products includes a wide range of properties including not only its softening ability but also temperature stability upon extrusion, resistance to migration, cost, odor, biodegradability, and manufacturing and consumer safety. Typical plasticizing agents include citrates, phthalates, stearates, fats and oils. It is known that glycerol fatty acids, such as glycerol monostearate, stabilize cells by reducing the rate of gas diffusion from the cell. However, such glycerol fatty acids are unable to provide sufficient wettability.
There is thus a need or desire for a soft, flexible, low-density, open-cell, thermoplastic, absorbent foam, and a method of making such foam.